1. Field of the Invention
The present invention relates to a method of and apparatus for driving, in accordance with a recording signal, a thermal head including a plurality of aligned heating elements.
2. Description of the Prior Art
In a prior art thermal recording apparatus with a thermal head of the above-mentioned type, a current pulse signal being at a high level or a low level is supplied as a recording signal to heating elements to selectively cause them to heat so that a desired pattern can be displayed on a thermal recording paper. When performing one line of recording, the heating elements of a thermal head are usually divided into a number of groups and the groups are successively enabled in consideration of the scale of a driving circuit, the capacity of a power supply for recording and the thermal characteristics of the thermal head. However, on the one hand, the number of heating elements included in each group which are enabled simultaneously in accordance with the recording signal is predetermined irrespective of an information pattern of the recording signal, and on the other hand, the capacity of the power supply for recording is designed to match the maximum number of the heating elements which are simultaneously caused to heat within one group. Therefore, due to the fact that the probability that the maximum number of the heating elements in each group are simultaneously caused to heat is extremely small in the course of practical recording, the scale of the recording apparatus unnecessarily becomes large to disadvantage.
This tendency is notable especially in high speed thermal recording apparatus. A solution to this problem has been proposed, for example, in Japanese Patent Application Laid-open No. 56544/77 (laid open to public on May 10, 1977), wherein heating elements included in each group are further divided into a number of blocks, which number is varied dependent on the number of high levels in a recording signal portion associated with each group, and the blocks are then successively enabled.
With this proposal, however, the division into the groups is fixed and irrespective of the distribution of high levels in the recording signal and therefore, even when the rate of appearance of the high levels is not so large, the number of divisions into the blocks may be too large to thereby prevent effective increase in the recording speed.